Reducing errors and adverse events have become a central focus of the health care system over the last two decades. However, patients with mental disorders have been systematically excluded from this research. As a result, the epidemiology of patient safety events (adverse events and errors) in hospital based mental health services remains unknown. The current application seeks to assess the incidence, nature and preventability of patient safety events via a record review of 11,000 Medicaid patient medical charts in a random sample of 38 inpatient psychiatric units of general hospitals in Pennsylvania. We will supplement this information with detailed surveys of unit leadership to define the patient, provider, and psychiatric unit/hospital factors that influence, contribute to, and/or protect against the commission of adverse events and/or errors. In-depth qualitative interviews with key informants from ten hospitals will be used to help interpret these quantitative findings and more deeply understand the mechanisms by which patient, provider and unit factors interact and contribute to cause harm and error with an eye towards intervention development.